1. Field of the Invention
The present invention relates to a color image forming device which scans light beams on a photoreceptor for a plurality of colors to thereby form electrostatic latent images, develops the thus formed electrostatic latent images in a plurality of color toners, and transfers the plurality of images to the same recording medium such as a sheet of recording paper to thereby form a color image and, in particular, to a color image forming device which is capable of restricting color positional deviation or color shifting between the transferred images on the recording medium.
2. Description of the Related Art
For a color image forming device which produces a color image by transferring a plurality of different color toner images on a sheet of recording paper superimposingly, there are conventionally known several transfer methods: for example, in one method, the electrostatic latent images of a manuscript image are formed by each of a plurality of colors on a drum-shaped or belt-shaped photoreceptor by use of light beams, the thus formed electrostatic latent images are developed in their respective color toners, and the toner images are transferred or sequentially superimposed on each other on a sheet of recording paper; or, in another method, a plurality of electrostatic latent images each having its own color different from others are formed in a plurality of photoreceptors respectively provided for respective colors, the electrostatic latent images are developed in their own colors, and the developed images are transferred or superimposed on each other on a sheet of recording paper.
In the above-mentioned types of conventional transfer methods, however, if the electrostatic latent images each having one of the plurality of colors are not superimposed on each other on the same sheet accurately, then the colors are caused to shift from each other in the resultant color image, failing to provide a color image of high quality.
In view of this, as a technique for superimposing the electrostatic latent images on each other in order to prevent such color shifting, there is employed a technique which detects the scanning start position of the light beams on the photoreceptors for forming the electrostatic latent images of the manuscript image and the transporting position of the sheet of recording paper, and then makes these positions coincide with each other at the transfer point.
FIG. 8 is an explanatory view of a light beam radiation block of a light beam radiation mechanism employed in a conventional color image forming device, and FIG. 9 is a timing chart of the light beam radiation in the conventional color image forming device shown in FIG. 8. In these figures, reference character SOS designates a light beam scan start signal on the photoreceptor, TR0 designates a detection signal occurring when a sheet of recording paper to be transported to a transfer position passes through a fixed reference position, and PS designates a write start signal for the electrostatic image. As shown in FIGS. 8 and 9, the write start signal PS is output in the following manner: that is, the SOS signal input after the paper reference position detection signal TR0 is generated is counted by a counter 50, and the write start signal PS is generated at a stage when the count value reaches a given value and is input to a laser driver 51, whereby an image signal is output from a laser 52.
Therefore, as shown in FIG. 9, the timing of generation of the paper reference position detection signal TR0 may vary every transfer due to shift in the timing of transporting the sheet of recording paper, which may in turn vary a time t.sub.e which is a period of time necessary from the generation of the detection signal TR0 to the start of counting of the SOS signal.
The variations of the time t.sub.e lead to the shifting of the write start position of the image in the sub-scanning direction thereof on the sheet of recording paper, resulting in the shifted colors in a color image to be formed.